In many applications it is necessary to use band-pass filters which provide selected amplitude and phase characteristics. Thus, in many cases it is desirable that the amplitude vs. frequency characteristics have a substantially flat response (a substantially constant and relatively high gain) in the relatively narrow pass band region about a center resonant frequency, f.sub.r, and be extremely small (relatively low gain) at all other frequencies, effectively approaching zero at extreme off-resonant frequencies. It is further desirable that the phase shift be zero over the narrow pass band region.
A band-pass amplifier-filter configuration which has found success in the prior art is described in my copending application, Ser. No. 422,733, entitled "Systems for Providing Digital Representations of Analog Values," filed concurrently with, and having the same filing date as, this application. The two stage band-pass amplifier-filter circuit described in such copending application utilizes a pair of series-cascased bi-quad amplifiers following a preamplifier circuit. Such circuit effectively provides extremely high open loop gain at the resonant frequency, i.e., at the center frequency of the pass band, which in the particular system discussed therein was selected at 384 Hz. The overall circuit is made completely stable under closed loop conditions through the use of a pair of resistors suitably arranged as described therein.
The band-pass circuit which is so described appears to provide satisfactory operation under normal conditions so long as the input signal thereto does not exceed a value at which the amplifier overloads. When overload occurs, however, the phase shift of the bi-quad amplifier becomes abnormal and the circuit becomes unstable during closed loop operation and is unable to recover from such instability and is thus not able to resume its normal desired operation even when the input signal falls below such overload level. The only effective way to cause the circuit to recover and to return to its normal operating state is to remove the excitation power supplied thereto and to let the components settle down before resuming a power-on condition. Such a procedure is clearly disadvantageous in most applications and it is desirable that the band-pass filter be designed so as to permit a graceful recovery following overload without the need to shut off power.